Spread spectrum OFDM (orthogonal frequency division multiplexing) multiple access, is one example of a spectrally efficient wireless communications technology. OFDM can be used to provide wireless communication services.
In OFDM spread spectrum system, the total spectral bandwidth is normally divided into a number of orthogonal tones, e.g. subcarrier frequencies. In a cellular network, the same bandwidth is often reused in all the cells of the system. Those tones hop across the bandwidth for the purpose of channel (frequency) diversity and interference averaging. Tone hopping follows predefined tone hopping sequences so that the hopped tones of a given cell do not collide with each other. The tone hopping sequences used in neighboring cells could be different to average interference between cells.
One exemplary form of the tone hopping sequences, is
                                          F            j                    ⁡                      (            t            )                          =                              SLOPE                          {                                                                    1                    j                                    ⁢                  mod                  ⁢                                                                          ⁢                  N                                +                t                            }                                ⁢                                          ⁢          mod          ⁢                                          ⁢          N                                    (        1        )            
In the above equation, N is the total number of the tones, t is the OFDM symbol index, j is the index of a tone hopping sequence, j=0, . . . , N-1, and Fj(t) is the index of the tone occupied by the j-th tone hopping sequence at time t. SLOPE is a cell specific parameter that uniquely determines the tone hopping sequences used in a given cell. Neighboring cells could use different values of SLOPE.
Information (control and data) is transported via various physical channels. A physical channel corresponds to one or more tone hopping sequences defined in Equation (1). Therefore, those tone hopping sequences are sometimes referred to as data tone hopping sequences. In a physical channel, the basic transmission unit is a channel segment. A channel segment includes the tones corresponding to the data tone hopping sequence(s) of the data channel over some time interval usually corresponding to a number of OFDM symbols.
In addition to the data tone hopping sequences, the OFDM spread spectrum system may also use a pilot in a downlink to facilitate various operations, such operations may include synchronization and channel estimation. A pilot normally corresponds to one or more pilot tone hopping sequences. One exemplary form of a pilot tone hopping sequence, as disclosed in U.S. patent application Ser. No. 09/551,791, isPilotj(t)=SLOPE·t+Oj mod N  (2)
By using different values for SLOPE, different pilot sequences will occur. Different pilot sequences may be used in different cells.
In the above equation, N, t, and SLOPE are the same parameters as used in Equation (1), j is the index of a pilot tone hopping sequence, Pilotj(t) is the index of the tone occupied by the j-th pilot tone hopping sequence at time t, and Oj is a fixed offset number of the j-th pilot tone hopping sequence. Normally, the cells in a system use the same set of offsets {Oj}.
In the OFDM spread spectrum system, the pilot and data tone hopping sequences are normally periodic with the same periodicity and use the same value for parameter SLOPE. The time interval of one period of a tone hopping sequence is sometimes referred to as a super slot. Thus, a super slot corresponds to a period after which a pilot sequence will repeat. The structures of the pilot, physical channels, and channel segments generally repeat from one super slot to another, and therefore can be uniquely determined once the super slot boundaries have been identified.
FIG. 1 shows a frequency vs time graph 100 used to illustrate general concepts of data and pilot tone hopping sequences, control and data traffic channels, channel segments, and super slots.
FIG. 1 includes a first row 102, a second row 104, a third row 106, a fourth row 108, and a fifth row 110. Each row 102, 104, 106, 108, 110 corresponds to a different orthogonal frequency tone in the frequency domain.
FIG. 1 also includes a first column 112, a second column 114, a third column 116, a fourth column 118, a fifth column 120 a sixth column 122, a seventh column 124, an eighth column 126, a ninth column 128, and a tenth column 130. Each column 112, 114, 116, 118, 120, 122, 124, 126, 128, 130 corresponds to an OFDM symbol time in the time domain.
In the FIG. 1 example, super slots 133, 135 each have a period equal to the period of the tone hopping sequence. First super slot 133 has a period of five OFDM symbol times represented by first through fifth columns 112, 114, 116, 118, 120 and defined by vertical time domain boundary lines 111 and 121. Second super slot 135 also has a period of five OFDM symbol times. Super slot 135 corresponds to sixth through tenth columns 122, 124, 126, 128, 130 and is defined by vertical time domain boundary lines 121 and 131.
During the first super slot (columns 112, 114, 116, 118, 120), data tone hopping sequences are shown for a first traffic segment. Three tones are dedicated to the first traffic segment during each symbol period. The data tone hopping sequence for the first exemplary traffic channel segment is illustrated by diagonal line shading which descends in FIG. 1 from left to right. During the second super slot (columns 122, 124, 126, 128, 130), data tone hopping sequences are shown for a second traffic segment. The data tone hopping sequence repeats in each super slot 133, 135. The data tone hopping sequence for the second exemplary traffic channel segment is illustrated by ascending diagonal line shading in FIG. 1. During the OFDM time intervals represented by first column 112 and the sixth column 122, the traffic channel data is shown to include frequency tones represented by first row 102, second row 104 and third row 106. During the OFDM time intervals represented by second column 114 and the seventh column 124, the traffic channel data is shown to include frequency tones represented by first row 102, third row 106 and fifth row 110. During the OFDM time intervals represented by third column 116 and the eighth column 126, the traffic channel data is shown to include frequency tones represented by second row 104, fourth row 108 and fifth row 110. During the OFDM time intervals represented by fourth column 118 and the ninth column 128, the traffic channel data is shown to include frequency tones represented by first row 102, third row 106 and fourth row 108. During the OFDM time intervals represented by fifth column 120 and the tenth column 130, the traffic channel data is shown to include frequency tones represented by second row 104, third row 106 and fourth row 108.
FIG. 1 also shows a pilot tone hopping sequence. The pilot tone hopping sequence repeats in each super slot 133, 135. The pilot tone hopping sequence is illustrated in FIG. 1 by use of small horizontal line shading. During the OFDM time intervals represented by first column 112 and the sixth column 122, the pilot tone is assigned to the frequency tone represented by fifth row 110. During the OFDM time intervals represented by second column 114 and the seventh column 124, the pilot tone is assigned to the frequency tone represented by fourth row 108. During the OFDM time intervals represented by third column 116 and the eighth column 126, the pilot tone is assigned to the frequency tone represented by third row 106. During the OFDM time intervals represented by fourth column 118 and the ninth column 128, the pilot tone is assigned to the frequency tone represented by the second row 104. During the OFDM time intervals represented by fifth column 120 and the tenth column 130, the pilot tone is assigned to the frequency tone represented by the first row 102.
In some OFDM spread spectrum systems, the traffic channel is assigned in a segment-by-segment manner. Specifically, traffic channel segments can be independently assigned to different wireless terminals. A scheduler determines the amount of transmission power and the burst data rate, associated with a particular channel coding and modulation scheme, to be used in each traffic channel segment. The transmission powers and burst data rates of different traffic channel segments may be different.
Sectorization is a popular method to improve wireless system capacity. For example, FIG. 2 illustrates a cell 200 including three sectors: sector 1 201, sector 2 203, and sector 3 205. Cell 200 also includes a base station 207 employing a 3-sector antenna including antenna sector 1 209, antenna sector 2 211, and antenna sector 3 213. The sectorized antenna provides some isolation between the sectors 201, 203, 205. In an ideal system, the same spectrum can be reused in all the sectors 201, 203, 205 without interfering with each other, thereby tripling the system capacity (over an omni cell) in the 3-sector system shown in FIG. 2. Unfortunately, ideal signal separation is not possible in the real world, which generally complicates the use of sectorization in some systems.
In theory, integrating the sectorization into an OFDM spread spectrum system should improve the overall system performance. However interference between the sectors due to the limited antenna isolation and reflection from objects can limit the actual capacity gains over an omni cell. Accordingly, it can be appreciated that there is a need for methods and apparatus which will allow sectorization to be used in OFDM systems in a manner that will improve the capacity of such systems without many of the interference problems associated with sectorization.